1. Field of the Invention
The present invention relates to a method for the denitrification and desulfurization of hot waste gases, particularly from furnaces. The method includes initially conducting a crude gas flow through a denitrification plant and through the endothermic part of a heat-utilizing plant. Subsequently, the flow is conducted into a desulfurization plant. The purified gas flow discharged from the desulfurization plant is conducted through the exothermic part of the heat-utilizing plant and is subsequently transferred to ambient.
The present invention also relates to an arrangement for the denitrification and desulfurization of hot waste gases, particularly from furnaces. The arrangement includes a crude gas flow conduit which is conducted through a denitrification plant and through the endothermic part of a heat-utilizing plant. The crude gas flow conduit is also connected to a desulfurization plant. A purified gas flow conduit downstream of the desulfurization plant is conducted through the exothermic part of the heat-utilizing plant.
2. Description of the Related Art
Large coal power plants are operated today almost exclusively with desulfurization plants for waste gases. During the combustion process in the power plant, the sulfur from the coal is converted to sulfur dioxide SO.sub.2. The sulfur dioxide is removed from the waste gases in the desulfurization plant to prevent it from reaching the atmosphere. In the atmosphere, the sulfur dioxide can oxidize to SO.sub.3 under the influence of sunlight and due to long-term effects. The SO.sub.3, in turn, reacts with the water vapor of the atmosphere to sulfuric acid and, thus, is responsible to a significant extent for damages to the environment due to acid, for example, damage to forests.
In addition to SO.sub.2, the combustion process also produces SO.sub.3 directly to a very small extent. The SO.sub..sub.3 is combined already within the combustion plant with water vapor to form H.sub.2 SO.sub.4 which is subjected together with the waste gases to the entire process and then reaches the atmosphere.
While SO.sub.2 is removed in the desulfurization plant to a high extent (usually over 90%), the degree of removal for SO.sub..sub.3 or H.sub.2 SO.sub.4 in vapor form is substantially poorer. It is usually lower than 50%.
In most cases, the desulfurization is performed by spraying or dripping the SO.sub..sub.2 -containing waste gases with milk of lime or similar washing solutions which contain absorbing agents.
Although the very small molecules of SO.sub..sub.3 or H.sub.2 SO.sub.4 in the form of gas or vapor pass through the desulfurization plant to a large extent without being removed, the effect on the environment is still low because only small amounts of these molecules are released during the normal combustion process.
More recently, waste gases (crude gases) from furnaces are not only desulfurized but also additionally denitrified. This means that the nitric oxide NO.sub.x which is contained in the waste gases and is produced by the combustion is also removed. This is because it has been found that NO.sub.x is converted to acids in the atmosphere under the influence of sunlight and, consequently, is also responsible for environmental damage.
The denitrification is carried out by adding ammonia NH.sub.3 to the waste gases. The ammonia converts the nitric oxide in a catalyst to molecular nitrogen N.sub.2 and water vapor H.sub.2 O.
Since significantly higher temperatures are required for this catalytic reaction than for the desulfurization, the denitrifying process is frequently carried out before the desulfurization. The waste gases to be purified arrive with the required high temperature of about 350.degree. C. directly from the boiler or furnace and are conducted after denitrification with an almost unchanged temperature directly to a heat-utilizing plant.
A heat-utilizing plant between the denitrification plant and the desulfurization plant is also required because the desulfurization plant requires temperatures of the waste gases for the washing process which are substantially below 100.degree. C.
A heat-utilizing plant which is frequently used is a rotating heat retainer which removes the heat energy from the waste gases which flow through hollow spaces of the heat retainer. The heat retainer stores and later releases the heat energy to the same or another gas flow, particularly to the purified gas which is discharged from the desulfurization plant.
As a result, the hot waste gases are cooled, while the part of the heat retainer which is in contact with the waste gases is heated. On the other hand, the cold purified gas is heated, while the part of the heat retainer which is in heat-exchanging contact with the purified gas is cooled. Such a rotating heat retainer is generally called a regenerator. Because of its purpose of later reheating the cold gas flow, the heat retainer is also called a gas preheater.
Since the waste gases to be desulfurized have to be cooled before entering the desulfurization plant and the purified gas must be heated again after leaving the desulfurization plant, so that moisture damage in the stack is avoided, it is apparent that the gas preheater should be used for this purpose, for cooling the waste gases in front of the desulfurization plant and for heating the purified gas following the desulfurization plant.
However, after the first denitrification plants to be arranged in front of a desulfurization plant were started to operate, it was found that now substantially more SO.sub..sub.3 was contained in the waste gases which reacted with the water vapor of waste gases to sulfuric acid. This substantially greater proportion of sulfuric acid in the waste gases resulted in a relatively high sulfuric acid dew point and, thus, in a precipitation of liquid sulfuric acid when the gas was cooled in the gas preheater in front of the desulfurization plant. This resulted in substantial corrosion of the gas preheater due to the precipitated liquid sulfuric acid and simultaneously in a contamination of the clean purified gas emerging from the desulfurization plant when passing through the gas preheater due to corrosion particles from the heat retainer.
Further tests have shown that the increased SO.sub..sub.3 portion in the waste gases is due to the catalytic effect of the NO.sub.x catalyst where a portion of the SO.sub.2 gas is catalytically converted to SO.sub..sub.3.
In addition to the significant corrosion damage to the gas preheater due to the high content of sulfuric acid, an increased portion of sulfuric acid travels through the desulfurization plant. This portion reaches the stack and, thus, is admitted to the atmosphere. Another portion of the liquid sulfuric acid precipitated at the gas preheater is again vaporized when the purified gas is reheated and, thus, travels also directly from the heat retainer of the gas preheater into the clean purified gas flow and through the stack into the atmosphere.
The high sulfuric acid content in the purified gas contradicts not only the previous efforts for cleaning the waste gases, but also leads relatively quickly to drop precipitation of the sulfuric acid-containing purified gas which is cooled quickly when leaving the stack and, thus, together with the corrosion particles from the gas preheater, leads to a direct and substantial burden on the immediate surroundings.
While it would theoretically be possible to prevent the SO.sub..sub.3 -conversion in the catalyst by means of completely new catalysts, this can only be a long term solution. In addition, such a development will require extremely high and unforeseeable costs.